Automatic Direction Finder (ADF) — Page 99, Lesson 82

Let’s pick this up with the outbound tracking picture, because that’s where the real technique starts to come together. When you’re flying away from an NDB — that’s the Non-Directional Beacon, the ground transmitter your ADF needle points at — you have to hold a specific track, not just a heading. In zero wind, with zero drift, it’s simple. Figure 7.14 shows the aircraft heading 260° with a relative bearing of 180°. Remember what relative bearing means: it’s the angle measured clockwise from the aircraft’s nose to the beacon. So if the beacon is dead astern, the relative bearing is 180°, and your heading equals your track. That’s the no-drift case. Now let’s add wind. Figure 7.15 shows an aircraft maintaining a track of 100° in crosswind, where the drift is already known. Here 23° of starboard drift is anticipated. Starboard means the wind is pushing you to the right. To hold the ground track, you must subtract that drift from the track. So 100° minus 23° gives you a heading of 077°, and the relative bearing from the NDB becomes 203°. Notice the relationship: with the beacon behind you, the relative bearing is no longer 180° once you’ve crabbed into wind. Figure 7.16 is the mirror image. Now 20° of port drift is anticipated — wind pushing you left. This time you add the drift, plus, to the track. So track 090° plus 20° gives a heading of 110°, and the relative bearing is 160°. So the rule you want to lock in: starboard drift is subtracted from track to get heading; port drift is added. And the relative bearing shifts accordingly — greater than 180° for starboard drift, less than 180° for port drift, when outbound. Now, what if you don’t know the drift? That’s where drift assessment comes in, and it works differently inbound versus outbound. Let me walk you through the inbound case first, because that’s Figure 7.17. Initially, you fly the aircraft on the required track with the beacon dead ahead — that’s 000° relative. You hold your heading steady and watch the relative bearing indicator. Here’s the key observation: if the relative bearing increases, the aircraft is experiencing port drift. Think about it — the needle is moving clockwise, meaning the beacon is drifting toward your right, which means your nose is being pushed left. So an increasing relative bearing tells you port drift. To regain track, you alter heading, say 30° starboard. As you turn toward the beacon, the relative bearing will decrease, and when you’ve regained the track, it will read 330°. Now you assume a likely drift — say 10° port — and calculate a new heading to maintain track. When you take up that corrected heading, the relative bearing becomes 350°. If your drift assessment was correct, that 350° relative bearing will hold steady all the way until you’re overhead the NDB. But if the relative bearing changes, you’ve misjudged the drift, and you’ll need further heading alterations and a fresh assessment. Now the outbound case, Figure 7.18. This is the reverse logic. With zero drift, the RBI — that’s the Relative Bearing Indicator — shows 180° relative, the beacon dead astern. With 10° starboard drift, the relative bearing increases to 190°. With 10° port drift, it decreases to 170°. So outbound, the drift direction flips the sign compared to inbound. And there’s a critical condition: to assess drift this way, the aircraft must maintain a steady heading from directly overhead the beacon. You have to be exactly over the NDB when you start, or the geometry is wrong. Once you’ve assessed the drift, you alter heading — port or starboard, by say 30° — to regain track. You hold that until the correct relative bearing of 210° or 150° is obtained. Why those numbers? Because with your 30° intercept angle, the relative bearing shifts by 30° from the 180° reference. When you see 210° or 150°, you’re back on track. Then, and only then, you alter heading again to take into account your original assessment of drift — that’s the crab angle you calculated. So the sequence outbound is: assess drift, intercept with a 30° heading change, regain track, then re-crab. Finally, let’s touch on holding, because it’s the practical use of all this. When traffic density or bad weather delays your landing, the air traffic controller directs you to a Holding Area. That area is also called a ‘stack’, and it’s organized over a radio beacon — typically an NDB. Each waiting aircraft flies a special circuit, and aircraft are separated vertically by a minimum of 1000 feet. As soon as you’re free of other traffic, you drop to the next level down, and you keep descending through the stack until you finally fly out and come in to land. Figure 7.20 shows that whole system — the stacked circuits over the beacon. So the thread through all of this: the relative bearing is your window into drift. Inbound, an increasing relative bearing means port drift. Outbound, an increasing relative bearing means starboard drift. And the correction technique — intercept with a 30° heading change, regain the track, then re-crab — is the same skill in both directions, just with the signs flipped.